Microalgae have become a remarkable novel industry of energy sources and nutrition. Since systematic sterilization is impossible in large-scale culture of microalgae, contamination is easily caused by harmful organisms. And such contamination usually results in a completely failure of the entire culture. The key of the invention is to provide a convenient and effective method for killing harmful organisms.
1. During the culture of algae, contamination caused by harmful organisms such as protozoa and other algae etc. commonly occurs. The harm caused by harmful organisms to the culture of microalgae is very severe, which often results in a complete failure of culture. The contamination and harm caused by harmful organisms is one of the major reasons that cause unstable quality of microalgae culture currently. And its controlling issue has not been solved till now.
2. Organism-Caused Harm to Culture of Microalgae
1) preying: harmful organisms such as Rotifera, Oxyrrhis sp, Amoeba sp, and etc. swallow algal cells directly. If the number of harmful organisms and their swallow amount are not large, their impact is not significant due to the fast reproduction rate of algal cells. However, when the number of harmful organisms and their swallow amount are increased, their harm to the culture of microalgae will become severe. In particular, Rotifera has a huge swallow amount. Algal cells can be eaten completely within 2-3 days, thereby resulting a clear algal liquid.
2) inhibiting and poisoning microalgae by secreting harmful substances: harmful organisms inhibit and poison algal cells by secreting certain harmful extracellular products, which is another aspect of the harm caused by harmful organisms and normally severer than direct swallowing. When harmful organisms secret a small amount of harmful substances, the cultured algae grow slowly. When harmful organisms reproduce in a large number and secret more harmful substances, a great deal of algal cells sink and die. The harmful substances secreted by harmful organisms have different toxic levels to different types of algae. The harm caused by a harmful organism maybe involves any one aspect or both aspects described above.
3. Strategies for Controlling Harmful Organisms
The principle of treating the contamination and harm caused by harmful organisms is mainly based on prevention or combination of prevention and control, i.e. reduction of their harmful level as low as possible.
1) Preventing Strategies
(1) In the process of culture, each production part is sterilized strictly and the environment is kept as sterile as possible, so as to prevent contamination from the initial source.
(2) Separating, culturing and supplying algae: in the process of culture, strict prevention of contamination is very important. However, from the point of view of the current technical level of microalgae culture, the contamination caused by harmful organisms is impossible to be prevented completely during a long-term cultivation. As a result, contamination occurs sooner or later, and harmful organisms can reproduce dramatically in a suitable condition and eventually result in the failure of culture. In order to solve this problem radically, the algae that have been contaminated during the long-term cultivation should be replaced by fresh pure algae constantly, which may enable the culture to go on wheels. Therefore, separation, culture and supply of algae are very important.
(3) Maintaining the growth and number dominance of the cultured algae: good growth of the cultured algae and their absolute number dominance in alga liquid are very important for preventing and alleviating the harm caused by harmful organisms. It has been taught by the experience in long-term cultivation that when the cultured algae grow well and reproduce rapidly, severe harm caused by harmful organisms occurs less frequently. Sometimes although the algae have been contaminated, harmful organisms is not increased, but decreased or even disappear after a period of cultivation. Therefore, maintenance of good growth of the cultured algae and their number dominance and inhibition of harmful organisms by secreting more extracellular product is one of the reasons that lead to success of culturing. To maintain algae to grow well, first, the inoculated algae must be obtained in the exponential growth phase and second, the environmental condition should satisfy the requirement for alga growth as close as possible and allow the algae to grow prosperously. The algae should be inoculated in a large amount. As a result, the algae have an absolute number dominance in the culture liquid from the beginning of cultivation.
2) Methods for eliminating, inhibiting or killing harmful organisms
The following three methods are usually used for eliminating, inhibiting or killing harmful organisms.
(1) Eliminating Large-Sized Harmful Organisms by Filtering
The large-sized contaminating harmful organisms (e.g. Rotifera, etc.) can be eliminated by filtering, because all bait algae are small. Typically, Rotifera can be filtered through a screening silk with meshes less than 60 um. After filtered once, adult Rotifera can be eliminated, but the eggs and the younger individuals under developing cannot been eliminated completely until completion of 3-daycontinuous filtering, once a day. When the eggs and younger individuals remained after the first round of filtering develop into adults, they can be eliminated in the second or third round of filtering.
(2) Inhibiting or Killing Harmful Organisms with Drugs
The drugs for inhibiting or killing harmful organisms are used.
(3) Killing Harmful Organisms by Changing Environmental Conditions
In order to use the method for killing harmful organisms by changing environmental conditions, ranges of the environmental factors such as temperature, salinity, pH etc. suitable for culturing algae and harmful organisms must be understood. Then according to a specific situation, a certain environmental factor can be altered to achieve the purpose of killing harmful organisms but preserving algae.